U.S. patent application number 15/317180 was filed with the patent office on 2017-05-18 for positive electrode active material for nonaqueous electrolyte secondary battery.
This patent application is currently assigned to SANYO Electric Co., Ltd.. The applicant listed for this patent is SANYO Electric Co., Ltd.. Invention is credited to Daizo Jito, Akihiro Kawakita, Takeshi Ogasawara.
Application Number | 20170141391 15/317180 |
Document ID | / |
Family ID | 55217018 |
Filed Date | 2017-05-18 |
United States Patent
Application |
20170141391 |
Kind Code |
A1 |
Jito; Daizo ; et
al. |
May 18, 2017 |
POSITIVE ELECTRODE ACTIVE MATERIAL FOR NONAQUEOUS ELECTROLYTE
SECONDARY BATTERY
Abstract
There is provided a positive electrode active material for a
nonaqueous electrolyte secondary battery capable of suppressing an
increase in DCR during cycles. There is provided a positive
electrode active material for a nonaqueous electrolyte secondary
battery that includes a secondary particle formed by aggregation of
primary particles formed of a lithium transition metal oxide. A
rare-earth compound secondary particle formed by aggregation of
particles formed of a rare-earth compound adheres to a recess
formed between primary particles adjacent to each other on a
surface of the secondary particle, and the rare-earth compound
secondary particle adheres to both the primary particles adjacent
to each other in the recess. A tungsten-containing compound adheres
to an interface of primary particles inside the secondary particle
formed of the lithium transition metal oxide.
Inventors: |
Jito; Daizo; (Osaka, JP)
; Ogasawara; Takeshi; (Hyogo, JP) ; Kawakita;
Akihiro; (Hyogo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SANYO Electric Co., Ltd. |
Daito-shi, Osaka |
|
JP |
|
|
Assignee: |
SANYO Electric Co., Ltd.
Daito-shi, Osaka
JP
|
Family ID: |
55217018 |
Appl. No.: |
15/317180 |
Filed: |
July 14, 2015 |
PCT Filed: |
July 14, 2015 |
PCT NO: |
PCT/JP2015/003550 |
371 Date: |
December 8, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/621 20130101;
H01M 4/131 20130101; H01M 2004/028 20130101; H01M 4/525 20130101;
H01M 4/485 20130101; H01M 4/362 20130101; Y02E 60/10 20130101; H01M
10/0525 20130101 |
International
Class: |
H01M 4/525 20060101
H01M004/525; H01M 4/62 20060101 H01M004/62; H01M 4/131 20060101
H01M004/131; H01M 4/485 20060101 H01M004/485 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 30, 2014 |
JP |
2014-154464 |
Claims
1. A positive electrode active material for a nonaqueous
electrolyte secondary battery, the positive electrode active
material comprising a secondary particle formed by aggregation of
primary particles formed of a lithium transition metal oxide,
wherein a rare-earth compound secondary particle formed by
aggregation of particles formed of a rare-earth compound adheres to
a recess formed between primary particles adjacent to each other on
a surface of the secondary particle, and the rare-earth compound
secondary particle adheres to both the primary particles adjacent
to each other in the recess, and a tungsten-containing compound
adheres to an interface of primary particles inside the secondary
particle formed of the lithium transition metal oxide.
2. The positive electrode active material for a nonaqueous
electrolyte secondary battery according to claim 1, wherein the
rare-earth compound contains a rare-earth element, and the
rare-earth element is at least one element selected from the group
consisting of neodymium, samarium, and erbium.
3. The positive electrode active material for a nonaqueous
electrolyte secondary battery according to claim 1, wherein the
rare-earth compound is at least one compound selected from the
group consisting of hydroxides and oxyhydroxides.
4. The positive electrode active material for a nonaqueous
electrolyte secondary battery according to claim 1, wherein the
tungsten-containing compound contains lithium.
5. The positive electrode active material for a nonaqueous
electrolyte secondary battery according to any claim 1, wherein a
proportion of nickel in the lithium transition metal oxide is 80%
or more based on a total molar quantity of metal elements other
than lithium.
6. The positive electrode active material for a nonaqueous
electrolyte secondary battery according to claim 1, wherein a
proportion of cobalt in the lithium transition metal oxide is 7 mol
% or less based on a total molar quantity of metal elements other
than lithium.
7. The positive electrode active material for a nonaqueous
electrolyte secondary battery according to claim 1, wherein the
rare-earth compound contains a rare-earth element, and the
rare-earth element is at least one element selected from the group
consisting of neodymium, samarium, and erbium, wherein the
tungsten-containing compound contains lithium, wherein a proportion
of nickel in the lithium transition metal oxide is 80% or more
based on a total molar quantity of metal elements other than
lithium.
8. The positive electrode active material for a nonaqueous
electrolyte secondary battery according to claim 1, wherein the
rare-earth compound contains a rare-earth element, and the
rare-earth element is at least one element selected from the group
consisting of neodymium, samarium, and erbium, wherein the
rare-earth compound is at least one compound selected from the
group consisting of hydroxides and oxyhydroxides, wherein the
tungsten-containing compound contains lithium, wherein a proportion
of nickel in the lithium transition metal oxide is 80% or more
based on a total molar quantity of metal elements other than
lithium.
9. The positive electrode active material for a nonaqueous
electrolyte secondary battery according to claim 7, wherein a
proportion of cobalt in the lithium transition metal oxide is 7 mol
% or less based on a total molar quantity of metal elements other
than lithium.
10. The positive electrode active material for a nonaqueous
electrolyte secondary battery according to claim 8, wherein a
proportion of cobalt in the lithium transition metal oxide is 7 mol
% or less based on a total molar quantity of metal elements other
than lithium.
Description
TECHNICAL FIELD
[0001] The present invention relates to a positive electrode active
material for nonaqueous electrolyte secondary batteries.
BACKGROUND ART
[0002] In recent years, nonaqueous electrolyte secondary batteries
have been required to have high capacity that allows long-term
operation and improved output characteristics in the case where
charge and discharge are repeatedly performed with a large current
within a relatively short time.
[0003] PTL 1 below suggests that when a group III element on the
periodic table is provided on surfaces of base particles serving as
a positive electrode active material, the reaction between the
positive electrode active material and an electrolytic solution can
be suppressed even in the case where the charge voltage is
increased, which suppresses the degradation of charge storage
characteristics.
[0004] PTL 2 below suggests that when a positive electrode active
material in which fine particles containing lithium tungstate are
formed on surfaces of primary particles is used, the initial
discharge capacity is increased, which reduces the resistance of a
positive electrode.
CITATION LIST
Patent Literature
[0005] PTL 1: International Publication No. 2005/008812
[0006] PTL 2: Japanese Published Unexamined Patent Application No.
2013-125732
SUMMARY OF INVENTION
Technical Problem
[0007] However, it has been found that the use of the techniques
disclosed in PTL 1 and PTL 2 still poses a problem in that the
direct current resistance (hereafter may be referred to as DCR)
after high-temperature cycles is increased, that is, the output
characteristics are degraded.
Solution to Problem
[0008] Accordingly, a positive electrode active material for a
nonaqueous electrolyte secondary battery according to the present
invention includes a secondary particle formed by aggregation of
primary particles formed of a lithium transition metal oxide. A
rare-earth compound secondary particle formed by aggregation of
particles formed of a rare-earth compound adheres to a recess
formed between primary particles adjacent to each other on a
surface of the secondary particle, and the rare-earth compound
secondary particle adheres to both the primary particles adjacent
to each other in the recess. A tungsten-containing compound adheres
to an interface of primary particles inside the secondary particle
formed of the lithium transition metal oxide.
Advantageous Effects of Invention
[0009] According to the present invention, there can be provided a
positive electrode active material for nonaqueous electrolyte
secondary batteries capable of suppressing an increase in DCR
during high-temperature cycles.
BRIEF DESCRIPTION OF DRAWINGS
[0010] FIG. 1 includes a cross-sectional view schematically
illustrating a positive electrode active material particle and a
partially enlarged cross-sectional view schematically illustrating
the positive electrode active material according to an embodiment
and Experimental Example 1 of the present invention.
[0011] FIG. 2 is a partially enlarged cross-sectional view
schematically illustrating a positive electrode active material in
Experimental Example 3.
[0012] FIG. 3 is a partially enlarged cross-sectional view
schematically illustrating a positive electrode active material in
Experimental Example 5.
[0013] FIG. 4 is a partially enlarged cross-sectional view
schematically illustrating a positive electrode active material in
Reference Experimental Example 1.
DESCRIPTION OF EMBODIMENTS
[0014] An embodiment of the present invention will be described
below. This embodiment is merely an example for carrying out the
present invention, and the present invention is not limited to the
embodiment and can be appropriately modified without changing the
spirit of the present invention. The drawings referred to in the
description of the embodiment and Experimental Examples are
schematically illustrated. The dimensions and amounts of
constituent elements in the drawings may be different from those of
actual elements.
[0015] A nonaqueous electrolyte secondary battery according to an
embodiment of the present invention includes a positive electrode
containing a positive electrode active material, a negative
electrode containing a negative electrode active material, a
nonaqueous electrolyte containing a nonaqueous solvent, and a
separator. For example, the nonaqueous electrolyte secondary
battery has a structure in which an electrode body obtained by
winding a positive electrode and a negative electrode with a
separator disposed therebetween and a nonaqueous electrolyte are
accommodated in a case.
[Positive Electrode]
[0016] A positive electrode active material includes a secondary
particle formed by aggregation of primary particles formed of a
lithium transition metal oxide. A rare-earth compound secondary
particle formed by aggregation of primary particles formed of a
rare-earth compound adheres to a recess formed between primary
particles adjacent to each other on a surface of the secondary
particle, and the rare-earth compound secondary particle adheres to
both the primary particles adjacent to each other in the recess. A
tungsten-containing compound adheres to an interface of primary
particles inside the secondary particle formed of the lithium
transition metal oxide.
[0017] Hereafter, the structure of the positive electrode active
material for nonaqueous electrolyte secondary batteries will be
described in detail with reference to FIG. 1. As illustrated in
FIG. 1, the positive electrode active material includes lithium
transition metal oxide secondary particles 21 formed by aggregation
of lithium transition metal oxide primary particles 20. Rare-earth
compound secondary particles 25 formed by aggregation of rare-earth
compound primary particles 24 adhere to recesses 23 formed between
lithium transition metal oxide primary particles 20 adjacent to
each other on the surfaces of the lithium transition metal oxide
secondary particles 21. Furthermore, the rare-earth compound
secondary particles 25 adhere to both the lithium transition metal
oxide primary particles 20 adjacent to each other in the recesses
23. In the positive electrode active material, a
tungsten-containing compound 27 adheres to interfaces of the
lithium transition metal oxide primary particles 20 inside the
lithium transition metal oxide secondary particles 21. The
tungsten-containing compound 27 preferably adheres to both primary
particles 20 adjacent to or facing each other.
[0018] In the above structure, since the rare-earth compound
secondary particles 25 adhere to both the lithium transition metal
oxide primary particles 20 adjacent to each other in the recesses
23, the surface alteration of the lithium transition metal oxide
primary particles 20 adjacent to each other during charge-discharge
cycles at high temperatures can be suppressed and also the cracking
at the interfaces of the primary particles in the recesses 23 can
be suppressed. In addition, the rare-earth compound secondary
particles 25 produce an effect of fixing (bonding) the lithium
transition metal oxide primary particles 20 adjacent to each other.
Therefore, even if the positive electrode active material is
repeatedly subjected to expansion and shrinkage during
charge-discharge cycles at high temperatures, the cracking at the
interfaces of the primary particles in the recesses 23 is
suppressed.
[0019] In the above structure, the tungsten-containing compound 27
adheres to the interfaces of the primary particles inside the
lithium transition metal oxide secondary particles 21 even at high
temperatures. Therefore, the surface alteration of the primary
particles inside the lithium transition metal oxide secondary
particles 21 and the cracking at the interfaces of the primary
particles are suppressed during charge-discharge cycles at high
temperatures. Furthermore, since the rare-earth compound secondary
particles 25 adhere to both the lithium transition metal oxide
primary particles 20 adjacent to each other in the recesses 23 of
the lithium transition metal oxide secondary particles 21, the
elution of the tungsten-containing compound 27 is suppressed even
at high temperatures.
[0020] In the above structure, as described above, the surface
alteration and cracking of the positive electrode active material
are suppressed on the surface of and inside the positive electrode
active material during charge-discharge cycles at high
temperatures.
[0021] The phrase "the rare-earth compound secondary particles
adhere to both the lithium transition metal oxide primary particles
adjacent to each other in the recesses" refers to a state in which,
when the cross-section of lithium transition metal oxide particles
is observed, the rare-earth compound secondary particles adhere to
both the surfaces of the lithium transition metal oxide primary
particles adjacent to each other in the recesses that are formed
between the lithium transition metal oxide primary particles
adjacent to each other on the surfaces of the lithium transition
metal oxide secondary particles.
[0022] The rare-earth compound is preferably at least one compound
selected from the group consisting of rare-earth hydroxides,
oxyhydroxides, oxides, carbonates, phosphates, and fluorides. Among
them, the rare-earth compound is particularly preferably at least
one compound selected from the group consisting of rare-earth
hydroxides and oxyhydroxides because such a rare-earth compound
produces a larger effect of suppressing the surface alteration
caused at the interfaces of the primary particles.
[0023] Examples of a rare-earth element in the rare-earth compound
include scandium, yttrium, lanthanum, cerium, praseodymium,
neodymium, samarium, europium, gadolinium, terbium, dysprosium,
holmium, erbium, thulium, ytterbium, and lutetium. Among them,
neodymium, samarium, and erbium are particularly preferred. This is
because compounds of neodymium, samarium, and erbium produce a
larger effect of suppressing the surface alteration caused at the
interfaces of the primary particles than other rare-earth
compounds.
[0024] Specific examples of the rare-earth compound include
hydroxides and oxyhydroxides such as neodymium hydroxide, neodymium
oxyhydroxide, samarium hydroxide, samarium oxyhydroxide, erbium
hydroxide, and erbium oxyhydroxide; phosphates and carbonates such
as neodymium phosphate, samarium phosphate, erbium phosphate,
neodymium carbonate, samarium carbonate, and erbium carbonate; and
oxides and fluorides such as neodymium oxide, samarium oxide,
erbium oxide, neodymium fluoride, samarium fluoride, and erbium
fluoride.
[0025] The average particle size of the rare-earth compound primary
particles is preferably 5 nm or more and 100 nm or less and more
preferably 5 nm or more and 80 nm or less.
[0026] The average particle size of the rare-earth compound
secondary particles is preferably 100 nm or more and 400 nm or less
and more preferably 150 nm or more and 300 nm or less. If the
average particle size is more than 400 nm, the particle size of the
rare-earth compound secondary particles is excessively increased,
which decreases the number of lithium transition metal oxide
recesses to which the rare-earth compound secondary particles
adhere. Consequently, there are many lithium transition metal oxide
recesses that are not protected by the rare-earth compound
secondary particles, which may reduce an effect of suppressing an
increase in DCR. If the average particle size is less than 100 nm,
the contact area of the rare-earth compound secondary particles
between the lithium transition metal oxide primary particles is
decreased. As a result, the effect of fixing (bonding) the lithium
transition metal oxide primary particles adjacent to each other is
reduced, which may reduce an effect of suppressing the cracking of
surfaces of the secondary particles at the interfaces of the
primary particles.
[0027] The average particle size of the lithium transition metal
oxide secondary particles is preferably 2 .mu.m or more and 40
.mu.m or less and more preferably 4 .mu.m or more and 20 .mu.m or
less. If the average particle size is less than 2 .mu.m, the
lithium transition metal oxide secondary particles are excessively
small as secondary particles and high density required for the
positive electrode is not achieved, which may make it difficult to
achieve high capacity. If the average particle size is more than 40
.mu.m, output at low temperatures is sometimes not sufficiently
obtained. The lithium transition metal oxide secondary particles
are formed by bonding (aggregation) of the lithium transition metal
oxide primary particles.
[0028] The average particle size of the lithium transition metal
oxide primary particles is preferably 100 nm or more and 5 .mu.m or
less and more preferably 300 nm or more and 2 .mu.m or less. If the
average particle size is less than 100 nm, the amount of interfaces
of the primary particles (including primary particles inside the
secondary particles) is excessively increased, which may easily
cause cracking due to expansion and shrinkage during cycles. If the
average particle size is more than 5 .mu.m, the amount of
interfaces of the primary particles (including primary particles
inside the secondary particles) is excessively decreased, which may
decrease the output at low temperatures. Since secondary particles
are formed by aggregation of primary particles, the lithium
transition metal oxide primary particles are never larger than the
lithium transition metal oxide secondary particles.
[0029] The content (coating mass) of the rare-earth compound is
preferably 0.005 mass % or more and 0.5 mass % or less and more
preferably 0.05 mass % or more and 0.3 mass % or less in terms of
rare-earth element relative to the total mass of the lithium
transition metal oxide. If the content is less than 0.005 mass %,
the amount of the rare-earth compound that adheres to the recesses
between the lithium transition metal oxide primary particles
decreases. Consequently, the above-described effect of the
rare-earth compound is not sufficiently produced, which may fail to
suppress an increase in DCR after cycles. If the content is more
than 0.5 mass %, the rare-earth compound not only covers portions
between the lithium transition metal oxide primary particles, but
also excessively covers the surfaces of the lithium transition
metal oxide secondary particles, which may degrade the initial
charge-discharge characteristics.
[0030] Examples of the tungsten-containing compound include
tungsten trioxide (WO.sub.3), tungsten dioxide (WO.sub.2), and
lithium tungstate. In particular, lithium tungstate is preferred
because lithium tungstate has higher lithium ion conductivity than
tungsten oxide. Examples of the lithium tungstate include
Li.sub.2WO.sub.4, Li.sub.4WO.sub.5, and Li.sub.6W.sub.2O.sub.9.
[0031] The tungsten-containing compound adheres to the interfaces
of the primary particles inside the lithium transition metal oxide
secondary particles and may also adhere to the interfaces of the
primary particles on the surfaces of the lithium transition metal
oxide secondary particles.
[0032] The content of the tungsten-containing compound is
preferably 0.1 mass % or more and 5.0 mass % or less and
particularly preferably 0.3 mass % or more and 3.0% or less in
terms of tungsten element relative to the total mass of the lithium
transition metal oxide. If the content of the tungsten compound is
less than 0.1 mass %, an effect of suppressing the surface
alteration of the primary particles inside the secondary particles
tends to be not sufficiently produced. If the content is 5.0 mass %
or more, the diffusion of lithium ions between the lithium
transition metal oxide and the electrolytic solution tends to be
inhibited. In this specification, the phrase "the content of the
tungsten-containing compound relative to the total mass of the
lithium transition metal oxide" refers to, when the whole
tungsten-containing compound is assumed to be present in the form
of tungsten, a ratio of the mass of the tungsten-containing
compound that adheres to the inside and surfaces of the lithium
transition metal oxide secondary particles to the total mass of the
lithium transition metal oxide.
[0033] The tungsten compound according to the present invention is
present at the interfaces of the primary particles inside the
secondary particles. For example, when nickel-cobalt-aluminum
oxide, lithium hydroxide, and tungsten oxide are mixed and fired,
tungsten is sometimes partially substituted with nickel or cobalt
to form solid solution. However, this state is not a state in which
the tungsten compound is present at the interfaces of the primary
particles in the present invention.
[0034] The lithium transition metal composite oxide preferably
contains Ni in an amount of 80% or more relative to the total
amount of the metal elements other than lithium from the viewpoints
of not only a further increase in the positive electrode capacity
but also ease of a proton exchange reaction at the interfaces of
the primary particles described later. That is, when the total
molar quantity of metals other than Li in the lithium transition
metal oxide is assumed to be 1, the proportion of nickel is
preferably 80% or more. Specific examples of the lithium transition
metal composite oxide include lithium-containing nickel-manganese
composite oxide, lithium-containing nickel-cobalt-manganese
composite oxide, lithium-containing nickel-cobalt composite oxide,
and lithium-containing nickel-cobalt-aluminum composite oxide. The
lithium-containing nickel-cobalt-aluminum composite oxide may have
a composition containing nickel, cobalt, and aluminum at a molar
ratio of, for example, 8:1:1, 82:15:3, or 94:3:3. They may be used
alone or in combination.
[0035] In the lithium transition metal composite oxide, the
proportion of cobalt in the lithium transition metal oxide is
preferably 7 mol % or less and more preferably 5 mol % or less
relative to the total molar quantity of metal elements other than
lithium. If the proportion of the cobalt is excessively small, the
structure readily changes during charge and discharge and cracking
tends to readily occur at the interfaces of the particles.
Therefore, the DCR during high-temperature cycles readily increases
in a lithium transition metal composite oxide whose cobalt
proportion is 7 mol % or less. When the rare-earth compound and the
tungsten-containing compound are caused to adhere to the lithium
transition metal composite oxide particles whose cobalt proportion
is 7 mol % or less as illustrated in FIG. 1, the surface alteration
and cracking of the lithium transition metal composite oxide
particles are suppressed on the surfaces of and inside the
particles, which considerably produces an effect of suppressing an
increase in the DCR.
[0036] In the lithium transition metal composite oxide with a Ni
proportion (Ni percentage) of 80% or more, the proportion of
trivalent nickel is high and thus a proton exchange reaction
between water and lithium in the lithium transition metal oxide
readily occurs in the water. Consequently, a large amount of LiOH
generated as a result of the proton exchange reaction appears on
the surfaces of the secondary particles from the inside of the
interfaces of the lithium transition metal oxide primary particles.
Thus, the alkali (OH.sup.-) concentration in portions between the
lithium transition metal oxide primary particles adjacent to each
other on the surfaces of the lithium transition metal oxide
secondary particles becomes higher than that in the surrounding
portions. Rare-earth compound primary particles are attracted by
alkali and aggregated in the recesses formed between the primary
particles, and are easily precipitated while forming secondary
particles. In the lithium transition metal composite oxide with a
Ni proportion of less than 80%, the proportion of trivalent nickel
is low, which makes it difficult to cause the proton exchange
reaction. Therefore, the alkali concentration in portions between
the lithium transition metal oxide primary particles is
substantially the same as that in the surrounding portions. Thus,
even in the case where the precipitated rare-earth compound primary
particles are bonded to form secondary particles, when the
secondary particles adhere to the surface of the lithium transition
metal oxide, the secondary particles tend to adhere to protruding
portions of the lithium transition metal oxide primary particles
with which the secondary particles are likely to collide.
[0037] The lithium transition metal oxide may contain other
additional elements. Examples of the additional elements include
boron (B), magnesium (Mg), aluminum (Al), titanium (Ti), chromium
(Cr), iron (Fe), copper (Cu), zinc (Zn), niobium (Nb), molybdenum
(Mo), tantalum (Ta), zirconium (Zr), tin (Sn), tungsten (W), sodium
(Na), potassium (K), barium (Ba), strontium (Sr), calcium (Ca), and
bismuth (Bi).
[0038] The lithium transition metal oxide is preferably stirred in
a certain amount of water to remove an alkali component that
adheres to the surface of the lithium transition metal oxide from
the viewpoint of providing batteries having excellent
high-temperature storage characteristics.
[0039] In the production of the positive electrode active material
for nonaqueous electrolyte secondary batteries according to this
embodiment, a rare-earth compound is caused to adhere to the
surfaces of lithium transition metal oxide secondary particles and
then tungsten may be caused to adhere to the interfaces of primary
particles inside the lithium transition metal oxide secondary
particles, or tungsten is caused to adhere to the interfaces of
primary particles inside lithium transition metal oxide secondary
particles and then a rare-earth compound may be caused to adhere to
the surfaces of the lithium transition metal oxide secondary
particles. The rare-earth compound can be caused to adhere to the
surfaces of the lithium transition metal oxide secondary particles
by, for example, a method in which an aqueous solution containing a
rare-earth element is added to a suspension containing a lithium
transition metal oxide. Tungsten can be caused to adhere to the
interfaces of the primary particles inside the lithium transition
metal oxide secondary particles by, for example, a method in which
an aqueous solution containing tungsten is added to a lithium
transition metal oxide or a suspension containing a lithium
transition metal oxide.
[0040] When the rare-earth compound is caused to adhere to the
surfaces of the lithium transition metal oxide secondary particles,
the pH of the suspension is desirably adjusted to 11.5 or more and
preferably 12 or more while the aqueous solution in which the
compound containing a rare-earth element has been dissolved is
added to the suspension. Under this condition, rare-earth compound
particles are likely to unevenly adhere to the surfaces of the
lithium transition metal oxide secondary particles. If the pH of
the suspension is 6 or more and 10 or less, the rare-earth compound
particles evenly adhere to the entire surfaces of the lithium
transition metal oxide secondary particles, which may fail to
sufficiently suppress the cracking of an active material due to the
surface alteration that occurs at the interfaces of the primary
particles on the surfaces of the secondary particles. If the pH is
less than 6, at least part of the lithium transition metal oxide
may be dissolved.
[0041] The pH of the suspension is desirably adjusted to 14 or less
and preferably 13 or less. If the pH is more than 14, the size of
the rare-earth compound primary particles is excessively increased.
In addition, an excess amount of alkali is left in the lithium
transition metal oxide particles, which may cause gelation during
the preparation of slurry or may excessively generate gas during
the storage of batteries.
[0042] In the case where the aqueous solution in which the compound
containing a rare-earth element has been dissolved is added to the
suspension containing a lithium transition metal oxide, when the
aqueous solution is simply used, a rare-earth hydroxide is
precipitated. When a fluorine source is sufficiently added to the
suspension, a rare-earth fluoride is precipitated. When carbon
dioxide is sufficiently dissolved, a rare-earth carbonate is
precipitated. When phosphate ions are sufficiently added to the
suspension, a rare-earth phosphate is precipitated. Thus, the
rare-earth compound can be precipitated on the surfaces of the
lithium transition metal oxide particles. By controlling dissolved
ions in the suspension, for example, a rare-earth compound
including a hydroxide and a fluoride in a mixed manner is also
obtained.
[0043] The lithium transition metal oxide particles having surfaces
on which the rare-earth compound has been precipitated are
preferably heat-treated. The heat treatment temperature is
preferably 80.degree. C. or higher and 500.degree. C. or lower and
more preferably 80.degree. C. or higher and 400.degree. C. or
lower. If the heat treatment temperature is lower than 80.degree.
C., it may take an excessive time to sufficiently dry the positive
electrode active material obtained through the heat treatment. If
the heat treatment temperature is higher than 500.degree. C., a
part of the rare-earth compound that adheres to the surfaces
diffuses into the lithium transition metal composite oxide
particles, which may reduce an effect of suppressing the surface
alteration that occurs at the interfaces of the primary particles
on the surfaces of the lithium transition metal oxide secondary
particles. When the heat treatment temperature is 400.degree. C. or
lower, almost no rare-earth element diffuses into the lithium
transition metal composite oxide particles and the rare-earth
element firmly adheres to the interfaces of the primary particles,
which improves an effect of suppressing the surface alteration that
occurs at the interfaces of the primary particles on the surfaces
of the lithium transition metal oxide secondary particles and an
effect of bonding the primary particles. In the case where a
rare-earth hydroxide is caused to adhere to the interfaces of the
primary particles, most of the hydroxide changes into an
oxyhydroxide at about 200.degree. C. to about 300.degree. C., and
furthermore changes into an oxide at about 450.degree. C. to about
500.degree. C. Therefore, when the heat treatment is performed at
400.degree. C. or lower, a rare-earth hydroxide or oxyhydroxide
that produces a large effect of suppressing surface alteration can
be selectively provided to the interfaces of the lithium transition
metal oxide primary particles, which produces a good effect of
suppressing the DCR.
[0044] The lithium transition metal oxide having a surface on which
the rare-earth compound has been precipitated is preferably
heat-treated in a vacuum. The reason for this is as follows. The
moisture of the suspension used when the rare-earth compound is
caused to adhere penetrates to the inside of the lithium transition
metal oxide particles. When the rare-earth compound secondary
particles adhere to the recesses formed at the interfaces of the
primary particles on the surfaces of the lithium transition metal
oxide secondary particles, moisture is not easily removed from the
inside during the drying. Therefore, the moisture is not
effectively removed unless the heat treatment is performed in a
vacuum. This increases the amount of moisture brought into a
battery from the positive electrode active material. Consequently,
a product generated as a result of a reaction of the moisture and
an electrolyte may alter the surface of the active material.
[0045] The lithium transition metal oxide to which the tungsten
compound has adhered is preferably heat-treated in a vacuum. The
reason for this is the same as above. The moisture is not
effectively removed unless the heat treatment is performed in a
vacuum. This increases the amount of moisture brought into a
battery from the positive electrode active material. Consequently,
a product generated as a result of a reaction of the moisture and
an electrolyte may alter the surface of the active material.
Furthermore, when the heat treatment is performed in a vacuum, the
tungsten compound is absorbed into the secondary particles, and
thus can be efficiently provided to the interfaces of the primary
particles inside the secondary particles.
[0046] The aqueous solution containing a rare-earth element can be
prepared by dissolving a substance such as an acetate, a nitrate, a
sulfate, an oxide, or a chloride in water or an organic solvent.
Such a substance is preferably dissolved in water because high
solubility is achieved. In particular, when a rare-earth oxide is
used, an aqueous solution prepared by dissolving a rare-earth
sulfate, chloride, or nitrate that is prepared by dissolving the
rare-earth oxide in an acid such as sulfuric acid, hydrochloric
acid, nitric acid, or acetic acid can also be used because such an
aqueous solution is equivalent to the above aqueous solution
prepared by dissolving a compound in water.
[0047] When the rare-earth compound is caused to adhere to the
surfaces of the lithium transition metal oxide secondary particles
by a method in which the lithium transition metal oxide and the
rare-earth compound are mixed with each other in a dry process, the
rare-earth compound particles randomly adhere to the surfaces of
the lithium transition metal oxide secondary particles, which makes
it difficult to cause the rare-earth compound particles to
selectively adhere to the interfaces of the primary particles on
the surfaces of the secondary particles. When the method using a
dry process is employed, the rare-earth compound does not firmly
adhere to the lithium transition metal oxide, and thus an effect of
fixing (bonding) the primary particles is not produced.
Furthermore, when a positive electrode mixture is prepared by
adding a conductive agent, a binding agent, and the like, the
rare-earth compound is easily separated from the lithium transition
metal oxide.
[0048] The positive electrode active material is not limited to the
case where the above-described positive electrode active material
particles are used alone. The above-described positive electrode
active material may be used in combination with other positive
electrode active materials. The positive electrode active material
is not particularly limited as long as it is a compound capable of
reversibly intercalating and deintercalating lithium ions. Examples
of the compound include compounds having a layered structure and
being capable of intercalating and deintercalating lithium ions
while a stable crystal structure is maintained, such as lithium
cobaltate and lithium-nickel-cobalt-manganese oxide, compounds
having a spinel structure, such as lithium-manganese oxide and
lithium-nickel-manganese oxide, and compounds having an olivine
structure. When only positive electrode active materials of the
same type are used or when different types of positive electrode
active materials are used, the positive electrode active materials
may have the same particle size or different particle sizes.
[0049] A positive electrode containing the above positive electrode
active material suitably includes a positive electrode current
collector and a positive electrode mixture layer formed on the
positive electrode current collector. The positive electrode
mixture layer preferably contains a binding agent and a conductive
agent, in addition to the positive electrode active material
particles. The positive electrode current collector is formed of,
for example, a conductive thin film such as a metal foil or alloy
foil of aluminum or the like which is stable in the potential range
of a positive electrode or a film including a metal surface layer
made of aluminum or the like.
[0050] The binding agent may be, for example, a fluoropolymer or a
rubber polymer. Examples of the fluoropolymer include
polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and
modified products of the foregoing. Examples of the rubber polymer
include ethylene-propylene-isoprene copolymers and
ethylene-propylene-butadiene copolymers. They may be used alone or
in combination of two or more. The binding agent may be used
together with a thickener such as carboxymethyl cellulose (CMC) or
polyethylene oxide (PEO).
[0051] The conductive agent may be, for example, a carbon material
such as carbon black, acetylene black, Ketjenblack, graphite, or
vapor-grown carbon fiber (VGCF). They may be used alone or in
combination of two or more.
[Negative Electrode]
[0052] A negative electrode is produced by, for example, mixing a
negative electrode active material and a binding agent with water
or an appropriate solvent, applying the resulting mixture to a
negative electrode current collector, and drying and rolling the
negative electrode current collector. The negative electrode
current collector is suitably formed of, for example, a conductive
thin film such as a metal foil or alloy foil of copper or the like
which is stable in the potential range of a negative electrode or a
film including a metal surface layer made of copper or the like.
The binding agent may be, for example, PTFE as in the case of the
positive electrode, but is preferably a styrene-butadiene copolymer
(SBR) or a modified product thereof. The binding agent may be used
together with a thickener such as CMC.
[0053] Any negative electrode active material capable of reversibly
occluding and releasing lithium ions can be used. Examples of the
negative electrode active material include carbon materials, metals
such as Si and Sn and alloy materials that form alloys with
lithium, and metal oxides such as SiO.sub.x (0<X<2). These
negative electrode active materials may be used alone or in
combination of two or more.
[Nonaqueous Electrolyte]
[0054] A solvent of the nonaqueous electrolyte is, for example, a
cyclic carbonate, a chain carbonate, or a cyclic carboxylate.
Examples of the cyclic carbonate include propylene carbonate (PC)
and ethylene carbonate (EC). Examples of the chain carbonate
include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and
dimethyl carbonate (DMC). Examples of the cyclic carboxylate
include .gamma.-butyrolactone (GBL) and .gamma.-valerolactone
(GVL). These nonaqueous solvents may be used alone or in
combination of two or more.
[0055] A solute of the nonaqueous electrolyte is, for example,
LiPF.sub.6, LiBF.sub.4, LiCF.sub.3SO.sub.3, LiN(FSO.sub.2).sub.2,
LiN(CF.sub.3SO.sub.2).sub.2, LiN(C.sub.2F.sub.5SO.sub.2).sub.2,
LiN(CF.sub.3SO.sub.2) (C.sub.4F.sub.9SO.sub.2),
LiC(C.sub.2F.sub.5SO.sub.2).sub.3, or LiAsF.sub.6. Alternatively, a
lithium salt containing an oxalato complex as an anion may also be
used. Examples of the lithium salt include LiBOB [lithium
bisoxalate borate], Li[B(C.sub.2O.sub.4)F.sub.2],
Li[P(C.sub.2O.sub.4)F.sub.4], and
Li[P(C.sub.2O.sub.4).sub.2F.sub.2]. These solutes may be used alone
or in combination of two or more.
[Separator]
[0056] A known separator may be used. Examples of the separator
include polypropylene separators, polyethylene separators,
polypropylene-polyethylene multilayer separators, and separators
whose surface is coated with a resin such as an aramid resin.
[0057] A layer formed of a known inorganic filler may be formed at
an interface between the positive electrode and the separator or at
an interface between the negative electrode and the separator.
Examples of the filler include oxides and phosphates containing one
or more elements such as titanium, aluminum, silicon, and
magnesium; and those obtained by surface-treating the oxides and
phosphates with a hydroxide or the like.
EXAMPLES
[0058] Hereafter, Experimental Examples in Description of
Embodiments are used to more specifically describe the present
invention in detail. The present invention is not limited to
Experimental Examples below, and can be appropriately modified
without changing the spirit of the present invention.
First Experimental Example
Experimental Example 1
[Production of Positive Electrode Active Material]
[0059] LiOH and an oxide obtained by oxidizing, at 500.degree. C.,
a nickel-cobalt-aluminum composite hydroxide represented by
Ni.sub.0.94Co.sub.0.03Al.sub.0.03(OH).sub.2 and obtained by
coprecipitation were mixed with each other in an Ishikawa grinding
mixer so that the molar ratio of Li and all transition metals was
1.05:1. Subsequently, the resulting mixture was heat-treated in an
oxygen atmosphere at 760.degree. C. for 20 hours and then
pulverized to obtain lithium-nickel-cobalt-aluminum composite oxide
particles having an average secondary particle size of about 15
.mu.m and represented by
Li.sub.1.05Ni.sub.0.94Co.sub.0.03Al.sub.0.03O.sub.2.
[0060] To 1.5 L of pure water, 1000 g of the thus-obtained
lithium-nickel-cobalt-aluminum composite oxide particles serving as
a lithium transition metal oxide were added and stirred to prepare
a suspension in which the lithium transition metal oxide was
dispersed in the pure water. Subsequently, an aqueous erbium
sulfate solution prepared by dissolving erbium oxide in sulfuric
acid and having a concentration of 0.1 mol/L was added to the
suspension a plurality of times. The pH of the suspension was
maintained at 11.5 to 12.0 while the aqueous erbium sulfate
solution was added to the suspension.
[0061] The suspension was then filtered to obtain a powder. An
aqueous solution (hereafter, this solution may be referred to as a
tungsten aqueous solution in Experimental Examples) prepared by
dissolving 59 g of tungsten oxide (WO.sub.3) and 24 g of lithium
hydroxide (anhydride) in 460 ml of pure water was sprayed onto the
powder. The powder was dried in a vacuum at 200.degree. C. to
produce a positive electrode active material.
[0062] The surface of the positive electrode active material was
observed with a scanning electron microscope (SEM). It was
confirmed that erbium hydroxide secondary particles having an
average particle size of 100 to 200 nm and formed by aggregation of
erbium hydroxide primary particles having an average particle size
of 20 to 30 nm adhered to the surfaces of the lithium transition
metal oxide secondary particles. It was also confirmed that almost
all the erbium hydroxide secondary particles adhered to recesses
formed between the lithium transition metal oxide primary particles
adjacent to each other on the surfaces of the lithium transition
metal oxide secondary particles so as to be in contact with both
the primary particles adjacent to each other in the recesses.
Furthermore, the coating mass of the erbium compound was measured
by inductively coupled plasma (ICP) emission spectrometry. The
coating mass was 0.15 mass % relative to the
lithium-nickel-cobalt-aluminum composite oxide in terms of erbium
element.
[0063] The cross-section of the positive electrode active material
was observed with a scanning electron microscope (SEM). It was
confirmed that a tungsten compound was present at the interfaces of
the primary particles inside the secondary particles. The coating
mass of the tungsten compound was measured by inductively coupled
plasma (ICP) emission spectrometry. The coating mass was 0.67 mass
% relative to the lithium-nickel-cobalt-aluminum composite oxide in
terms of tungsten element.
[0064] In Experimental Example 1, it is believed that the pH of the
suspension is as high as 11.5 to 12.0, and thus the secondary
particles are formed as a result of bonding (aggregation) of the
erbium hydroxide primary particles precipitated in the suspension.
In Experimental Example 1, the Ni proportion is as high as 94%,
which increases the proportion of trivalent nickel. This
facilitates proton exchange between LiNiO.sub.2 and H.sub.2O at the
interfaces of the lithium transition metal oxide primary particles,
and a large amount of LiOH generated as a result of the proton
exchange reaction appears from the inside of the interfaces between
the primary particles adjacent to each other on the surfaces of the
lithium transition metal oxide secondary particles. This increases
the alkali concentration in portions between the primary particles
adjacent to each other on the surface of the lithium transition
metal oxide. Thus, the erbium hydroxide particles precipitated in
the suspension are attracted by alkali and aggregated in the
recesses formed at the interfaces of the primary particles, and
precipitated while forming secondary particles.
[Production of Positive Electrode]
[0065] The positive electrode active material particles, carbon
black serving as a conductive agent, and an N-methyl-2-pyrrolidone
solution in which polyvinylidene fluoride serving as a binding
agent was dissolved were weighed so that the mass ratio of the
positive electrode active material particles, the conductive agent,
and the binding agent was 100:1:1. They were kneaded using a T.K.
HIVIS MIX (manufactured by PRIMIX Corporation) to prepare a
positive electrode mixture slurry.
[0066] Subsequently, the positive electrode mixture slurry was
applied onto both surfaces of a positive electrode current
collector formed of an aluminum foil, dried, and then rolled with a
reduction roller. An aluminum current collecting tab was attached
thereto to produce a positive electrode plate including positive
electrode mixture layers formed on both surfaces of the positive
electrode current collector. The packing density of the positive
electrode active material in the positive electrode was 3.60
g/cm.sup.3.
[Production of Negative Electrode]
[0067] Artificial graphite serving as a negative electrode active
material, CMC (sodium carboxymethyl cellulose) serving as a
dispersant, and SBR (styrene-butadiene rubber) serving as a binding
agent were mixed at a mass ratio of 100:1:1 in an aqueous solution
to prepare a negative electrode mixture slurry. Subsequently, the
negative electrode mixture slurry was uniformly applied onto both
surfaces of a negative electrode current collector formed of a
copper foil, dried, and rolled with a reduction roller. A nickel
current collecting tab was attached thereto to produce a negative
electrode plate including negative electrode mixture layers formed
on both surfaces of the negative electrode current collector. The
packing density of the negative electrode active material in the
negative electrode was 1.75 g/cm.sup.3.
[Preparation of Nonaqueous Electrolytic Solution]
[0068] Lithium hexafluorophosphate (LiPF.sub.6) was dissolved in a
mixed solvent prepared by mixing ethylene carbonate (EC), methyl
ethyl carbonate (MEC), and dimethyl carbonate (DMC) at a volume
ratio of 2:2:6 so that the concentration of LiPF.sub.6 was 1.3
mol/L. A nonaqueous electrolytic solution was prepared by
dissolving 2.0 mass % of vinylene carbonate (VC) in the above mixed
solvent.
[Production of Battery]
[0069] The thus-produced positive electrode and negative electrode
were wound around a winding core in a spiral fashion with a
separator disposed between the electrodes. Then, the winding core
was pulled out to produce a spiral electrode body. Subsequently,
the spiral electrode body was flattened to obtain a flat electrode
body. Then, the flat electrode body and the nonaqueous electrolytic
solution were inserted into an aluminum laminate case to produce a
battery A1. The battery has a thickness of 3.6 mm, a width of 35
mm, and a length of 62 mm. When the nonaqueous electrolyte
secondary battery was charged to 4.20 V and discharged to 3.0 V,
the discharge capacity was 950 mAh.
Experimental Example 2
[0070] A battery A2 was produced in the same manner as in
Experimental Example 1, except that the powder obtained after the
filtration was dried in a vacuum at 200.degree. C. without being
sprayed with the tungsten aqueous solution in the production of the
positive electrode active material in Experimental Example 1.
Experimental Example 3
[0071] A battery A3 was produced in the same manner as in
Experimental Example 1, except that a positive electrode active
material was produced in the same manner as in Experimental Example
1 except that the pH of the suspension was maintained at 9 while
the aqueous erbium sulfate solution was added to the suspension in
the production of the positive electrode active material in
Experimental Example 1. Herein, 10 mass % of an aqueous sodium
hydroxide solution was appropriately added to adjust the pH of the
suspension to 9.
[0072] The surface of the positive electrode active material was
observed with a SEM. It was confirmed that erbium hydroxide primary
particles having an average particle size of 10 nm to 50 nm
uniformly adhered to the entire surfaces (both protruding portions
and recesses) of the lithium transition metal oxide secondary
particles without forming secondary particles. The coating mass of
the erbium compound was measured by inductively coupled plasma
(ICP) emission spectrometry. The coating mass was 0.15 mass %
relative to the lithium-nickel-cobalt-aluminum composite oxide in
terms of erbium element.
[0073] The cross-section of the positive electrode active material
was observed with a scanning electron microscope (SEM). It was
confirmed that the tungsten compound was present at the interfaces
of the primary particles inside the secondary particles. The
coating mass of the tungsten compound was measured by inductively
coupled plasma (ICP) emission spectrometry. The coating mass was
0.67 mass % relative to the lithium-nickel-cobalt-aluminum
composite oxide in terms of tungsten element.
[0074] In Experimental Example 3, it is believed that the
precipitation rate of the erbium hydroxide particles in the
suspension is low because the pH of the suspension is set to 9, and
thus the erbium hydroxide particles are uniformly precipitated on
the entire surfaces of the lithium transition metal oxide secondary
particles without forming secondary particles.
Experimental Example 4
[0075] A battery A4 was produced in the same manner as in
Experimental Example 1, except that the powder obtained after the
filtration was dried in a vacuum at 200.degree. C. without being
sprayed with the tungsten aqueous solution in the production of the
positive electrode active material in Experimental Example 3.
Experimental Example 5
[0076] A battery A5 was produced in the same manner as in
Experimental Example 1, except that the aqueous erbium sulfate
solution was not added and thus erbium hydroxide was not caused to
adhere to the surfaces of the lithium transition metal oxide
secondary particles in the production of the positive electrode
active material in Experimental Example 1.
[0077] The cross-section of the positive electrode active material
was observed with a scanning electron microscope (SEM). It was
confirmed that the tungsten compound was present at the interfaces
between the primary particles inside the secondary particles. The
coating mass of the tungsten compound was measured by inductively
coupled plasma (ICP) emission spectrometry. The coating mass was
0.67 mass % relative to the lithium-nickel-cobalt-aluminum
composite oxide in terms of tungsten element.
Experimental Example 6
[0078] A battery A6 was produced in the same manner as in
Experimental Example 1, except that the aqueous erbium sulfate
solution was not added and thus erbium hydroxide was not caused to
adhere to the surfaces of the lithium transition metal oxide
secondary particles, and also the tungsten aqueous solution was not
sprayed in the production of the positive electrode active material
in Experimental Example 1.
Experiment
[Measurement of DCR]
[0079] The DCRs of the produced batteries A1 to A6 before
charge-discharge cycles and after 100 cycles were measured under
the following conditions.
<Measurement of DCR Before Cycles>
[0080] After charge was performed at a current of 475 mA to an SOC
of 50%, constant voltage charge was performed at a battery voltage
at which the SOC reached 50% until the current reached 30 mA. The
OCV was measured 120 minutes after the completion of the charge.
Discharge was performed at 475 mA for 10 seconds to measure a
voltage after 10 seconds of discharge. The DCR (SOC 50%) before
cycles was determined from formula (1) below.
DCR(.OMEGA.)=(OCV(V) after 120 minutes of pause-voltage (V) after
10 seconds of discharge)/(current (A)) (1)
[0081] Subsequently, 100 charge-discharge cycles each including
charge and discharge under the following conditions were repeatedly
performed. The pause time between the measurement of the DCR before
cycles and the charge-discharge cycle test was 10 minutes.
<Charge-Discharge Cycle Test>
[0082] Charge Conditions
[0083] Constant current charge was performed at a current of 475 mA
until the battery voltage reached 4.2 V (the positive electrode
potential was 4.3 V with respect to lithium). After the battery
voltage reached 4.2 V, constant voltage charge was performed at a
constant voltage of 4.2 V until the current reached 30 mA.
[0084] Discharge Conditions
[0085] Constant current discharge was performed at a constant
current of 950 mA until the battery voltage reached 3.0 V.
[0086] Pause Conditions
[0087] The pause interval between the charge and the discharge was
10 minutes.
<Measurement of DCR after 100 Cycles>
[0088] The DCR after 100 cycles was measured by the same method as
that for measuring the DCR before cycles. The pause time between
the charge-discharge cycle test and the measurement of the DCR
after cycles was 10 minutes.
[0089] The measurement of DCR and the charge-discharge cycle test
were performed in a thermostat at 60.degree. C.
[Calculation of DCR Increase Rate]
[0090] The DCR increase rate after 100 cycles was calculated from
formula (2) below. Table 1 shows the results. DCR increase rate
(SOC 50%)
(DCR(SOC 50%) after 100 cycles)/(DCR(SOC 50%) before
cycles).times.100 (2)
TABLE-US-00001 TABLE 1 Adhesion state of Presence of Rare-earth
rare-earth tungsten DCR increase Battery element compound compound
rate (%) A1 Er aggregated in Yes 35 recess A2 Er aggregated in No
41 recess A3 Er uniformly Yes 48 dispersed A4 Er uniformly No 45
dispersed A5 -- -- Yes 48 A6 -- -- No 44
[0091] The battery A1 will be considered below. In the positive
electrode active material of the battery A1, the rare-earth
compound secondary particles 25 adhere to both the lithium
transition metal oxide primary particles 20 adjacent to each other
in the recesses 23 as illustrated in FIG. 1. This is believed to
suppress surface alteration and cracking at the interfaces of the
primary particles on both the surfaces of the adjacent lithium
transition metal oxide primary particles 20 during high-temperature
charge-discharge cycles. Furthermore, it is believed that the
rare-earth compound secondary particles 25 also produce an effect
of fixing (bonding) the lithium transition metal oxide primary
particles 20 adjacent to each other, which suppresses cracking at
the interfaces of the primary particles in the recesses 23.
[0092] In the battery A1, since the rare-earth compound secondary
particles 25 adhere to both the lithium transition metal oxide
primary particles 20 adjacent to each other in the recesses 23, the
elution of the tungsten-containing compound 27 from the inside of
the lithium transition metal oxide secondary particles 21 is
suppressed even at high temperatures. In the battery A1, therefore,
the tungsten-containing compound 27 adheres to the interfaces of
the primary particles inside the lithium transition metal oxide
secondary particles 21 even at high temperatures. This is believed
to suppress the surface alteration of the primary particles inside
the lithium transition metal oxide secondary particles 21 and the
cracking at the interfaces of the primary particles.
[0093] In the battery A1, as described above, the surface
alteration and cracking of the positive electrode active material
are suppressed on the surface of and inside the positive electrode
active material, and an increase in the resistance of the positive
electrode is suppressed. Thus, the DCR increase rate after the
high-temperature cycles was believed to be the lowest.
[0094] The batteries A3 and A5 will be considered below. In the
positive electrode active material used for the battery A3, the
rare-earth compound primary particles 24 uniformly adhere to the
entire surfaces of the lithium transition metal oxide secondary
particles 21 without forming secondary particles as illustrated in
FIG. 2. In the positive electrode active material used for the
battery A3, the tungsten-containing compound 27 adheres to the
interfaces of the primary particles inside the lithium transition
metal oxide secondary particles 21. In the positive electrode
active material used for the battery A5, as illustrated in FIG. 3,
a rare-earth compound does not adhere to the surfaces of the
lithium transition metal oxide secondary particles 21, and the
tungsten-containing compound 27 adheres to the interfaces of the
primary particles inside the lithium transition metal oxide
secondary particles 21.
[0095] In the batteries A3 and A5, the rare-earth compound
secondary particles do not adhere to the recesses 23 on the
surfaces of the lithium transition metal oxide secondary particles
21. Therefore, it is believed that the surface alteration of the
adjacent lithium transition metal oxide primary particles 20 and
the cracking at the interfaces of the primary particles cannot be
suppressed. It is also believed in the batteries A3 and A5 that the
rare-earth compound secondary particles 25 do not adhere to the
recesses 23, and thus the elution of the tungsten-containing
compound 27 from the inside of the lithium transition metal oxide
secondary particles 21 cannot be suppressed at high
temperatures.
[0096] The elution of the tungsten-containing compound 27
eliminates an effect of suppressing the alteration at the
interfaces of the primary particles inside the lithium transition
metal oxide secondary particles 21 and increases the resistance of
the positive electrode. Furthermore, a part of the eluted
tungsten-containing compound 27 is deposited on the surface of the
negative electrode, which increases the resistance of the negative
electrode. It is believed that since the elution of the
tungsten-containing compound 27 increases the resistances of the
positive electrode and the negative electrode, the DCR increase
rate after the high-temperature cycles is higher in the batteries
A3 and A5 than in the batteries A4 and A6 in which the
tungsten-containing compound 27 is not contained.
[0097] The batteries A2, A4, and A6 will be considered. The
positive electrode active materials for the batteries A2, A4, and
A6 respectively correspond to the positive electrode active
materials illustrated in FIGS. 1, 2, and 3, except that the
tungsten-containing compound 27 does not adhere to the positive
electrode active materials.
[0098] In the battery A2, the rare-earth compound secondary
particles 25 adhere to both the lithium transition metal oxide
primary particles 20 adjacent to each other in the recesses 23.
Thus, it is believed that the surface alteration and the cracking
at the interfaces of the primary particles on both the surfaces of
the lithium transition metal oxide primary particles 20 adjacent to
each other can be suppressed for the same reason as the battery A1.
In the battery A2, however, a tungsten-containing compound does not
adhere to the inside of the lithium transition metal oxide
secondary particles 21, and thus the surface alteration of the
primary particles inside the lithium transition metal oxide
secondary particles 21 and the cracking at the interfaces of the
primary particles cannot be suppressed. Therefore, it is believed
that the resistance of the positive electrode increases and the DCR
increase rate after the high-temperature cycles is higher in the
battery A2 than in the battery A1.
[0099] In the batteries A4 and A6, the rare-earth compound
secondary particles do not adhere to the recesses 23 on the
surfaces of the lithium transition metal oxide secondary particles
21, and thus the surface alteration of the adjacent lithium
transition metal oxide primary particles 20 and the cracking at the
interfaces of the primary particles cannot be suppressed.
Furthermore, in the batteries A4 and A6, a tungsten-containing
compound does not adhere to the inside of the lithium transition
metal oxide secondary particles 21, and thus the surface alteration
of the primary particles inside the lithium transition metal oxide
secondary particles 21 and the cracking at the interfaces of the
primary particles cannot be suppressed. Therefore, it is believed
that the resistance of the positive electrode increases compared
with the battery A2 and the DCR increase rate after the
high-temperature cycles is higher in the batteries A4 and A6 than
in the battery A2.
Second Experimental Example
Reference Example 1
[0100] LiOH and an oxide obtained by oxidizing, at 500.degree. C.,
a nickel-cobalt-manganese composite hydroxide represented by
Ni.sub.0.35Co.sub.0.35Mn.sub.0.30(OH).sub.2 and obtained by
coprecipitation were mixed with each other in an Ishikawa grinding
mixer so that the molar ratio of Li and all transition metals was
1.05:1. Subsequently, the resulting mixture was heat-treated in the
air at 1000.degree. C. for 20 hours and then pulverized to obtain a
lithium-nickel-cobalt-manganese composite oxide having an average
secondary particle size of about 15 .mu.m and represented by
Li.sub.1.05Ni.sub.0.35Co.sub.0.35Mn.sub.0.30O.sub.2.
[0101] A positive electrode active material was produced in the
same manner as in Experimental Example 1, except that the
lithium-nickel-cobalt-manganese composite oxide represented by
Li.sub.1.05Ni.sub.0.35Co.sub.0.35Mn.sub.0.30O.sub.2 was used
instead of the lithium-nickel-cobalt-aluminum composite oxide
represented by Li.sub.1.05Ni.sub.0.94Co.sub.0.03Al.sub.0.03O.sub.2
in Experimental Example 1. Thus, a positive electrode active
material in which erbium compound particles adhere to the surfaces
of the lithium transition metal oxide secondary particles was
produced.
[0102] The surface of the positive electrode active material was
observed with a SEM. It was confirmed that erbium hydroxide
secondary particles having an average particle size of 100 to 200
nm and formed by aggregation of erbium hydroxide primary particles
having an average particle size of 20 nm to 30 nm adhered to the
surfaces of the lithium transition metal oxide secondary particles.
It was also confirmed in the positive electrode active material
produced in Reference Example 1 that, as illustrated in FIG. 4, the
rare-earth compound secondary particles 25 formed by aggregation of
the rare-earth compound primary particles 24 adhered to protruding
portions 26 on the surfaces of the lithium transition metal oxide
secondary particles and only one of the lithium transition metal
oxide primary particles 20 adjacent to each other in the recesses
23 between the lithium transition metal oxide primary particles.
The coating mass of the erbium compound was measured by inductively
coupled plasma (ICP) emission spectrometry. The coating mass was
0.15 mass % relative to the lithium-nickel-cobalt-aluminum
composite oxide in terms of erbium element.
[0103] In Reference Example 1, the Ni proportion is as lows as 35%,
which decreases the proportion of trivalent nickel. Therefore, it
is believed that LiOH generated as a result of a proton exchange
reaction hardly appears on the surfaces of the lithium transition
metal oxide secondary particles through the interfaces of the
lithium transition metal oxide primary particles. In Reference
Example 1, the pH of the suspension is as high as 11.5 to 12.0 and
secondary particles are easily formed as a result of bonding
(aggregation) of erbium hydroxide primary particles precipitated in
the suspension. However, it is believed that when the erbium
hydroxide secondary particles adhere to the surface of the lithium
transition metal oxide, unlike Experimental Example 1, almost all
of the erbium hydroxide secondary particles adhere to the
protruding portions on the surfaces of the lithium transition metal
oxide secondary particles with which they are likely to collide.
Some of the erbium hydroxide secondary particles may adhere to the
recesses. However, in this case, the erbium hydroxide secondary
particles adhere to only one of the lithium transition metal oxide
primary particles adjacent to each other in the recesses.
[0104] In Experimental Examples above, erbium was used as a
rare-earth element, but the cases where samarium and neodymium were
used as rare-earth elements were also studied.
Third Experimental Example
Experimental Example 7
[0105] A battery A7 was produced in the same manner as in
Experimental Example 1, except that a samarium sulfate solution was
used instead of the aqueous erbium sulfate solution in the
production of the positive electrode active material in
Experimental Example 1. The coating mass of the samarium compound
was measured by inductively coupled plasma (ICP) emission
spectrometry. The coating mass was 0.13 mass % relative to the
lithium-nickel-cobalt-aluminum composite oxide in terms of samarium
element.
Experimental Example 8
[0106] A battery A8 was produced in the same manner as in
Experimental Example 1, except that a neodymium sulfate solution
was used instead of the aqueous erbium sulfate solution in the
production of the positive electrode active material in
Experimental Example 1. The coating mass of the neodymium compound
was measured by inductively coupled plasma (ICP) emission
spectrometry. The coating mass was 0.13 mass % relative to the
lithium-nickel-cobalt-aluminum composite oxide in terms of
neodymium element.
[0107] The DCR increase rates after 100 cycles of the produced
batteries A7 and A8 were determined under the same conditions as
those in Experimental Example 1.
TABLE-US-00002 TABLE 2 Adhesion state of Presence of Rare-earth
rare-earth tungsten DCR increase Battery element compound compound
rate (%) A1 Er aggregated in Yes 35 recess A7 Sm aggregated in Yes
36 recess A8 Nd aggregated in Yes 36 recess
[0108] As is clear from Table 2, when samarium or neodymium, which
is the same as erbium in terms of rare-earth element, is used, the
DCR increase rate is also suppressed. Therefore, the DCR increase
rate is believed to be also suppressed when a rare-earth element
other than erbium, samarium, and neodymium is used.
REFERENCE SIGNS LIST
[0109] 20 lithium transition metal oxide primary particle [0110] 21
lithium transition metal oxide secondary particle [0111] 23 recess
[0112] 24 rare-earth compound primary particle [0113] 25 rare-earth
compound secondary particle [0114] 26 protruding portion [0115] 27
tungsten-containing compound
* * * * *